(a) Field of the Invention
The present invention relates to a power factor correction circuit. More particularly, it relates to a power factor correction circuit for solving a problem of a diode short-circuit occurring in a conventional power factor correction circuit, and a driving method thereof.
(b) Description of the Related Art
A switching mode power supply (SMPS) in which a power factor correction circuit is not used generates a pulse-type input current to make a high-level harmonic current flow to an electricity transmission line. The pulse-type current increases a loss of a power transmission line and a transformer rather than contributing to power transmission. Thus capacities of transmission power lines, transformer substations, and power stations are increased compared to the SMPS using the power factor correction circuit.
In order to solve the problem, movements for regulating the current harmonic, such as EN61000-3-2, have recently occurred in many countries, and a lot of SMPSs use the power factor correction circuit for satisfying regulations. The SMPS is a device for converting an input supply voltage to at least one DC output voltage, and is used in most home appliances such as computers, monitors, and televisions (TV). The SMPS uses a power factor correction circuit for compensating a power factor by controlling an input current to follow an input voltage. That is, the power factor correction circuit controls an externally applied input current to follow an input voltage and simultaneously converts an input AC voltage to a constant DC voltage.
The power factor correction circuit uses a boost topology that includes an inductor, and has various modes according to a state of a current flowing through the inductor. In a discontinuous conduction mode, a zero point of the current flowing through the inductor exists so that the current discontinuously flows, and in a continuous conduction mode, the current zero point does not exist so that the current continuously flows. A critical conduction mode operates at the boundary of the continuous conduction mode and the discontinuous conduction mode, and the current flowing through the inductor is increased after the current flowing through the inductor becomes zero. The ST L6562 is the most well-known integrated circuit (IC) of the power factor correction circuit in the critical conduction mode, and FAN7527B, TDA4862, TDA4863, MC33260, MC33262, UC3852, and SG6561 are also ICs of the power factor correction circuit in the critical conduction mode.
FIG. 1 schematically shows a general power factor correction circuit of a critical conduction mode.
In the power factor correction circuit, switching of a switch S is determined according to a time that a current flowing to an inductor L becomes zero, and the current flowing to the inductor L can be detected by detecting a voltage of the inductor L. When a diode D is short-circuited due to a defect in the power factor correction circuit, an input voltage Vin is connected to an output capacitor C through the inductor L such that the input voltage Vin and an output voltage Vout of the power factor correction circuit become the same. Since the input voltage Vin and the output voltage Vout are the same, a lateral voltage of the inductor L becomes zero. When the lateral voltage of the inductor L is zero in the power factor correction circuit, a turn-on time of the switch S cannot be determined so that the switch S is designed to be compulsorily turned on, in general. When the switch S is compulsorily turned on while the diode D is in the short-circuit state, energy charged in the output capacitor C is discharged through the switch S so that an excessive amount of current instantly flows because no circuit for controlling the current exists between the switch S and the capacitor C.
FIG. 2 compares a voltage generated in a normal condition and a voltage generated by sensing the current flowing to the switch when the diode D is short-circuited. A voltage Vocp, which is an overcurrent reference voltage, compulsorily turns off the switch when a switch current sensing voltage Vcs is higher than the voltage Vocp.
The power factor correction circuit generates a signal for turning off the switch when the switching current sensing voltage Vcs and the overcurrent reference voltage Vocp become the same. However, a time delay occurs until the switch is substantially turned off due to circuits existing in the middle of transmission of the signal to the switch.
As shown in FIG. 2, a switch current increase, r1 due to the delay is small in the normal condition. However, when the diode D is short-circuited, the switch current is radically increased for a short time delay because the switch current has a steep slope so that a current increase r2 is very large. When the diode D is short-circuited, temperature of the switch is increased and switching operation is repeated due to the excessive current so that the switch may be damaged. That is, a set protection circuit is required when the diode D is short-circuited.
In addition, the power factor correction circuit may use another diode D2 to charge the output capacitor C so as to prevent noise generated from charging through the inductor L and the diode D at the starting stage or from inductor saturation.
FIG. 3 shows a power factor correction circuit including a separate diode. In this case, when a diode D1 is short-circuited, an input voltage Vin and an output voltage Vout become the same and therefore energy stored in an inductor L is not entirely discharged when the switch S is turned off. In addition, the switch S is turned on before a current of the diode D1 current becomes zero so that a reverse recovery current of the diode D1 flows through the switch S. In general, in the case of a power factor correction circuit of the critical conduction mode, the switch S is turned on after a current of a diode D1 becomes zero and the diode D1 is turned off, and therefore a diode having a long reverse recovery time is used. That is, since the reverse recovery current of the diode D1 flows for a long period of time, the temperature of the switch S is increased, thereby causing damage to the switch S.
A conventional method for preventing damage to a switch includes a method for additionally including a diode, as shown in FIG. 4. When diodes D1 and D2 are short-circuited, a current flow from an output of the power factor correction circuit to an input thereof can be prevented by using additional diodes D3 and D4. However, in this case, the additional diodes D3 and D4 cause a cost increase and a diode's forward voltage drop increases loss, thereby deteriorating efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.